Laser has been simply used as a light source of various optical systems. Recently, sensors employing a change in properties of a laser such as frequency, polarization and phase, etc. in accordance with a change in external physical quantities, have been suggested.
Among them, a ring laser gyroscope has been applied in various fields (see: W. W. Chow et al., "The Ring Laser Gyro", Rev. of Mod. Phys., 57(1):61(1985)), which has advantages that: its structure is simple, intensity of optical signal is strong, and signal processing is easily made. As schematically depicted in FIG. 1, the said ring laser gyroscope comprises a ring laser resonator consisting of a laser gain medium(111) and reflecting mirrors(112, 113, 114).
If the laser gain medium in the ring laser gyroscope is activated by external energy, light is generated and amplified in the medium, and the laser oscillates. Since there are two laser beams(121, 122) with different directions in the resonator, one laser beam(121) and the other laser beam(122) circulate in clockwise and counterclockwise directions in the resonator, respectively. If the resonator rotates, the two beams circulating in the same and opposite direction against the direction of the resonator's rotation experience different optical path lengths, respectively, which is commonly known as "Sagnac effect". Owing to the Sagnac effect, the counterpropagating laser beams have different resonant conditions, thus the two laser beams have different frequencies whose difference is proportional to rotation rate. Portions of the two laser beams do not reflect on the reflecting mirror(114) and pass through it, which gives rise to laser output beams(123, 124). The laser output beams(123, 124) reflect on the reflecting mirrors(115, 116), respectively, thus meet at an optical beam splitter(117), which results in interference. On the other hand, a beam(125) moving in one direction from the optical beam splitter(117) has a signal corresponding to the said frequency difference, which is detected by an optical detector to measure rotation rate.
However, two beams generating from such a ring laser gyroscope are amplified in the same laser gain medium. Thus, gain competition, a phenonmenon that the intensity of a beam circulating in one direction decreases when the intensity of a beam circulating in the other direction increases, is caused, which gives rise to a shortcoming of unstability in the intensity of an output beam. Also, if rotation rate of the laser resonator is low, change in frequency caused by the rotation rate does not occur, which is called as lock-in effect, and the rotation rate can not be measured.
On the other hand, a passive fiber-optic gyroscope has been also reported in the art, as one of typical sensors to measure rotation rate. A general passive fiber-optic gyroscope is schematically depicted in FIG. 2(see: R. A. Bergh et al., "An Overview of Fiber-Optic Gyroscope", IEEE J. Lightwave Tech., LT-2:91(1984)), which permits measurement of magnetic field, current, etc., as well as rotation rate. It works on the principle that light from a light source(211) passes through directional couplers(212, 213), and propagates in clockwise and counterclockwise directions in the fiber loop, respectively. In this connection, light intensity measured in an optical detector(215) is proportional to (1+cos(.DELTA..phi.)) wherein .DELTA..phi. is a phase difference between the counterpropagating lights. If there is no nonreciprocal effect (rotation and magnetic field, etc.), a phase difference between two lights is 0. However, if there exists rotation or magnetic field, etc., two lights experience different optical path length during propagation around Sagnac loop (214). Therefore, output depends on the rate of rotation and the intensity of magnetic field. Accordingly, the rate of rotation and the intensity of magnetic field can be determined by measuring the intensity of an output light, and current can be determined considering on the magnetic field generated by the current.
However, the said passive fiber-optic gyroscope has revealed a serious problem that: if the counterpropagating lights have different states of polarization by a birefringence existing in an optical fiber, errors in measurement may occur(see: H. Lefevre, in "The Fiber-Optic Gyroscope", Artech House, p.73(1993)).
In order to solve the said problem, a method for constructing Sagnac loop employing a polarization maintaining fiber was suggested(see: U.S. Pat. No. 5,270,791). However, the method has not been practically applied in the art, since such a polarization maintaining fiber costs high.
Also, two directional couplers were employed between a light source and Sagnac loop in order to remove phase error generating in a directional coupler(see: U.S. Pat. No. 4,964,131). However, the prior art has also revealed a shortcoming that light intensity detected by an optical detector is reduced to below a quarter of intensity of light from a light source, since only a part of light from a light source(211) passes through a directional coupler(212) and is detected by an optical detector(215).